Nitric oxide is a colorless low-polarity gas with a boiling point of −152° C. It is sparsely soluble in liquids, has relatively low chemical reactivity and does not exhibit significant acidic or basic character. Therefore, a cost effective, environmentally friendly process for removing NO from a gas is problematic.
The most common high efficiency method of removing NO from gas streams is to use selective catalytic reduction (SCR) of NO by ammonia over a catalyst bed at high temperature. While effective, this method is expensive in capital and operating cost and has a large footprint.
The reaction of the NO with various chemical reactants has been widely reported in the literature. Due to the low reactivity of NO, strong reagents are required for reacting it into another species. For example, oxidizing or reducing agents such as potassium permanganate, sodium hypochlorite, hydrogen peroxide and even sodium sulfite have been cited. The oxidation of NO produces NO2, which is then absorbed into alkaline reagents such as caustic.
The NO molecule is able to form coordination complexes with various metal ions such as Fe++, Co++, Ni++, and Cu+. These complexes serve to activate the NO for reaction with milder, more convenient reagents, such as water, oxygen or sulfur dioxide (which in water solution exists as sulfite or bisulfite, depending on the pH).
Coordination complex formation by the NO, particularly with iron amine polycarboxylic acid complexes such as ferrous ethylenediaminetetraacetic acid complex (FeII EDTA), for absorption of NO has also been widely reported. This allows efficient absorption of the NO into an aqueous medium. However, since the FeII EDTA reagent is relatively expensive, the FeII EDTA.NO product resulting from NO absorption must be regenerated back to FeII EDTA in order to make the process economically practical. Since SO2 and NO often occur together, as in flue gases produced by the combustion of sulfur bearing fuels, the most common method suggested for regeneration is the use of SO2 dissolved in the medium to react the nitrosyl group of the complex to reduced ‘N,S products’ such as iminodisulfonate and hydroxylamine disulfonate salts, which then accumulate in the solvent.
The use of transition metal complexes for NO removal has several problems that must be addressed in the development of a practical process. One problem is that the metal may have a tendency to precipitate from solution, usually as the hydroxide, since a pH of about >4 is required for effective SO2 removal. The use of appropriate chelating (sequestering) agents is effective in preventing precipitation. However, the chelating agent has an influence on the kinetics and equilibrium of NO complex formation.
A further problem is that different oxidation states of the metal have varying complex formation tendencies with NO, so maintaining the metal in the preferred oxidation state is required. For instance, ferrous iron, Fe(II), is readily oxidized to ferric iron, Fe(III), which is inactive for nitrosyl complex formation, by oxygen which may be present in the feed gas or reportedly even by NO itself. In addition, N2O, which is a very potent greenhouse gas and therefore undesirable, may form under some reaction conditions, particularly at pH>7. Further, the metal chelate losses must be kept low. The metal nitrosyl complex should be regenerated in a reasonably rapid process to recover the active absorbent in small equipment with short residence time. Various schemes such as electrolysis and chemical reagents (including the sulfite/bisulfite ions) have been proposed. Finally, the end product from the NO must be dealt with. In Fe EDTA systems containing sulfite/bisulfite, a mixture of end products has been observed, including iminodisulfonate.